This invention relates to a system for detecting in a receiver of a radio communication link or system interference signals which are inevitably introduced into a plurality of communication signals transmitted to the receiver in a common frequency band.
In a conventional terrestrial microwave communication link, interference which occurs among communication signals carrying information signals during transmission of the communication signals in a common frequency band from a transmitting or origination station to a receiving or destination station is usually not serious because the communication signals are sufficiently separated from one another by directivity and/or polarization discrimination among antenna beams. The interference does, however, becomes serious in several instances, for example where use is made of a narrow angular directivity separation of less directivity discrimination among the antenna beams in order to more effectively use the frequencies; a case where the polarization discrimination is deteriorated by rain, and a case where use is made of a region in which scatter of the electromagnetic waves for the communication signals is serious. In these cases, it is necessary to detect the interferences in order to derive control signals for compensating for or cancelling the interference. Compensation of the interference is also necessary in a communication link comprising one or more repeater stations between the origination and the destination stations as will be exemplified hereunder with reference to one of several figures of the accompanying drawing. It is thus indispensable for compensation of the interference to detect, at a receiver of the destination or repeater station, interference signals produced during transmission of the communication signals between the origination and the destination stations. It is to be noted here that the receiver of a repeater station utilized in the disclosed system is different from an ordinary receiver in that it is unnecessary in the former to derive original information signals carried by the communication signals.
Also in a satellite communication link wherein communication signals of a common frequency band are separated from one another by polarization discrimination and repeated by a transponder carried by a satellite, cross-polarization interference is inevitable due to rain and other factors. Detection of the interference signals is again mandatory in a receiver of the transponder which serves as one or more repeater stations or of a destination ground station.
A system for detecting the interference signals produced among cross-polarized electromagnetic waves, such as a dextrorotary and a levorotary elliptically polarized wave, of a common frequency is disclosed in U.S. Pat. No. 3,735,266 issued to Noach Amitay, an assignor to Bell Telephone Laboratories, Incorporated, or in U.S. Pat. No. 4,090,137 issued to Shoji Soma and Ikuno Sato, assignors to the present assignee. Pilot carrier signals of different frequencies are transmitted as the respective cross-polarized waves. In a receiving station, the pilot carrier signals and interference signals occurring across the cross-polarized channels are detected for comparison between their respective phases or amplitudes to determine levels of the interference signals. The disclosed system is applicable in general to a communication link comprising a transmitting station and the receiving station and no repeater station although possibility of inclusion of one or more repeater stations is mentioned in the referenced U.S. Pat. No. For example, the proposed system is applicable in a satellite communication link only to a down link and not to a whole link comprising an up link and the down link. This is because frequency conversion is carried out in a repeater station.